Consistent animation & UX between ExpansionPanel and Tree?

[Kumfert]

Bug, feature request, or proposal:

feature request

What is the expected behavior?

ExpansionPanel has pleasant default animations when the panels expand and shrink and the expand_more/chevron_right indicators rotate. I expected the new tree widget to have a consistent user experience as the other Material components.

What is the current behavior?

The subtrees hide and reveal without animation, which is a more jarring UX. Also, the transition of the chevron doesn't animate. (Perhaps it could be made to with additional coding, but ExpansionPanel doesn't require it.)

What are the steps to reproduce?

Compare the sample pages between the Tree Widgets vs. Expansion Panel Widgets

What is the use-case or motivation for changing an existing behavior?

When I play with hide/show on the expansion panel and compare that to hide/show on a subtree, it just doesn't feel like a consistent user experience. Given (1) how much Material Design emphasizes motion to guide users through spatial relationships, and (2) how there are default animations for ExpansionPanel, Stepper, and Tabs, it really makes Tree appear the oddball out.

Which versions of Angular, Material, OS, TypeScript, browsers are affected?

Angular 6.0.7
Material 6.3.3

Is there anything else we should know?

[dalemanthei]

We have a need for this in our library based on material. Can the core team provide any guidance or suggestions on the best way to do this?

Maybe it could be a PR against the material or we could work out some of the solution in our own repo.

[laike9m]

Also, what's the best way to add roration animation for raw expand_more/chevron_right icon? My current code is like this, I didn't use any material component.

{{isExpanded() ? 'expand_more' : 'chevron_right'}}

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